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Newswise — Bacteria in the gut of the Anopheles gambiae mosquito inhibit infection of the insect with Plasmodium falciparum, the parasite that causes malaria in humans, according to researchers at the Johns Hopkins Bloomberg School of Public Health. Scientists with the Bloomberg School's Malaria Research Institute found that removing these bacteria, or microbial flora, with antibiotics made the mosquitoes more susceptible to Plasmodium infection because of a lack of immune stimulation. Their study is published in the May 8, 2009, edition of the journal PLoS Pathogens.

As part of the malaria transmission cycle, a mosquito acquires the malaria-causing parasite when it feeds on blood from an infected person. The parasite develops within the mosquito and can then be transmitted to another human when the mosquito feeds again.

"Our study suggests that the microbial flora of mosquitoes is stimulating immune activity that protects the mosquito from Plasmodium infection. The same immune factors that are needed to control the mosquito's infection from the microbes are also defending against the malaria parasite Plasmodium," said George Dimopoulos, PhD, senior author of the study and associate professor with Johns Hopkins Malaria Research Institute. "The interplay between bacteria and the mosquito's immune system may have significant implications for the transmission of malaria in the field where mosquitoes may be exposed to different types of bacteria in different regions. Theoretically, these bacteria could be introduced to the mosquitoes to boost their immunity to the malaria parasite and make them resistant and incapable of spreading the disease. Our current research aims at identifying those bacteria that trigger the strongest mosquito immune defense against the malaria parasite."

As part of the study, the Johns Hopkins researcher treated mosquitoes with antibiotics to kill the gut bacteria. Treated mosquitoes were more susceptible to infection by Plasmodium when feeding on infected blood compared to mosquitoes that were not treated with antibiotics. To further verify the results, bacteria-free mosquitoes were infected with bacteria to determine if they were less susceptible to Plasmodium infection.

In addition, the researchers determined that mosquitoes infected with bacteria died earlier than mosquitoes without bacteria when infected with Plasmodium; 60 percent of the mosquitoes with gut-bacteria died compared to 40 percent of those free of bacteria—even with Plasmodium levels five times higher than those with bacteria.

"The malaria parasite must live in the mosquito for about two weeks in order to complete its life cycle and be transmitted to a person. The fact that these bacteria shorten the mosquito's life span is additional good news," said Dimopoulos.

Malaria kills over one million people worldwide each year; the majority of deaths are among children living in Africa.

The study on the "Implication of the mosquito midgut microbiota in the defense against malaria parasites" was published by Yuemei Dong, Fabio Manfredini and George Dimopoulos. The research was supported by the Johns Hopkins Malaria Research Institute and the National Institute for Allergy and Infectious Diseases (NIAID), National Institutes of Health.